1. Field of the Invention
This invention relates to alkali-metal-free phosphate glasses and more specifically to glasses that exhibit a temperature coefficient of refractive index (xcex2=dn/dT) close to zero for use in fiber amplifiers.
2. Description of the Related Art
With the deployment of the optical network, erbium doped fiber amplifiers (EDFAs) are in high demand because they enable the periodic optical reamplification of the optical signal. This amplification is required to balance the small absorption in the silica single mode fibers used to carry the optical signal. In standard EDFAs, optical gain and amplification are provided by doping the fiber glass with rare-earth dopants such as Er. The concentration of these dopants varies with the nature and composition of the glass matrix. There has been a strong incentive to dope Er into silica-based fibers to maintain their compatibility, and connectivity with standard silica telecommunication fibers using fusion splicing. However, in silica-based glass the concentration of Er has to be kept small to prevent quenching and clustering of the dopant, resulting in low gain per unit length. Due to limited gain per unit length, standard EDFAs are comprised of several meters of doped silica-based fibers. Fiber management increases the cost of the amplifier and its footprint.
In contrast to silica-based glasses, multi-component glasses can accommodate much higher rare-earth dopants leading the higher optical gain per unit length. Multi-component glasses are comprised of a glass network former, glass network intermediators, and glass network modifiers. Multi-component glasses based on a phosphate glass former have been developed for use in solid-state lasers and are described for instance in U.S. Pat. No. 4,333,848. To increase the absorption of the pump light, Er doped glasses are co-doped with Yb. In this case, the Yb atoms get excited by absorbing photons from the pump light and they transfer their energy to Er atoms. Yb/Er phosphate glasses were used in lasers, where as gain medium they were placed inside a cavity. Such lasers are described for instance in Jiang 1998, xe2x80x9cEr3+ doped phosphate glasses and lasers,xe2x80x9d J. of Non-crystalline solids 239, p. 143. The glasses were shaped into rods with a diameter of 5 mm.
To operate efficiently in flashlamp-pumped lasers 10 of the type shown in FIG. 1a, the active medium (the phosphate glass rod 12 of length l), which is positioned between reflectors 14 and 16 to form a laser cavity, must be uniformly and effectively pumped by an incoherent broad spectrum pump 18 to provide a homogeneous gain profile. The optical gain must exceed the optical losses of the cavity to reach the laser threshold and subsequent laser emission 20. For stable laser emission the length L of the optical path length of the cavity must be kept constant during laser operation. Since flashlamps have a spectrally broad emission spectrum 22 (see FIG. 1b), only a small fraction of the photons get absorbed by the gain medium (here the Yb ions). This situation is very different from that of erbium doped fiber amplifiers (EDFAs) in which a coherent spectrally narrow laser 24 is used to pump the gain medium. The broadband and incoherent nature of the light provided by flashlamps has an important consequence: to reach the threshold for population inversion and sufficient gain the total intensity of the flashlight is very high and its infra-red part of the spectrum that does not overlap with the Yb optical transition generates a lot of heat in the gain medium.
Hence, to get effectively pumped from the side (see cross-sectional view FIG. 1c) with the photons delivered by the flashlamp the phosphate glass rod must contain a high concentration of Yb. Likewise, since the power delivered by a flashlamp is limited, the Er concentration in a laser has to be kept relatively low. This is because the threshold for population inversion of Er in co-doped Yb/Er glasses is increasing with concentration of Er. For instance, Myers in U.S. Pat. No. 4,962,067 teaches that (column 5 lines 38-42) xe2x80x9cIt will be appreciated by those skilled in the art that sensitization in this manner allows a relatively low erbium concentration to be utilized such that the necessary population inversion can be more readily achieved.xe2x80x9d
When pumped with flashlamps, solid-state lasers are always subjected to a lot a heat. Since the phosphate glass rods get heated substantially, they expand. This expansion changes the optical path 26 of the cavity as illustrated in FIG. 1d. The optical path OP for the laser cavity shown in FIG. 1a is given by:
OP=nGl+nair(Lxe2x88x92l) xe2x80x83xe2x80x83(1)
where nG is the refractive index of the glass, nair is the refractive index of air, l is the length of the glass rod and L the total length of the cavity.
Due to heating, the optical path in the cavity is changed by two effects that occur simultaneously: 1) thermal expansion of the phosphate glass rod, and 2) a change of its refractive index. The thermal expansion is characterized by the coefficient of linear thermal expansion xcex1 given by:                     α        =                              1                          l              0                                ⁢                                    ⅆ              l                                      ⅆ              T                                                          (        2        )            
where l0 is the length of the glass rod at room temperature. The change in refractive index is characterized by the temperature coefficient of refractive index xcex2 given by:                     β        =                              ⅆ            n                                ⅆ            T                                              (        3        )            
The change in optical path length is characterized by the thermal coefficient of optical path length w given by:
w=xcex2+(nGxe2x88x92nair)xcex1xe2x80x83xe2x80x83(4)
Since xcex1 is a positive coefficient, it becomes clear from Eq. (4) that a negative value for the temperature coefficient of refractive index xcex2 is required to keep the optical path constant in the laser cavity. Thus, the composition of Yb/Er doped phosphate glasses for lasers must be adjusted to provide for a negative value of xcex2.
A negative value of xcex2 means that the refractive index of the glass decreases when the temperature is increased. For instance, Myers in U.S. Pat. No. 4,962,067 teaches that (column 5 lines 52-56) xe2x80x9cThe laser glasses of the present invention are relatively athermal and demonstrate a negative change in refractive index with temperature which nearly compensates for their positive coefficient of thermal expansion.xe2x80x9d
Yb/Er co-doped glasses have also been proposed in fiber amplifiers. A schematic representation of a fiber amplifier 30 is given in FIG. 2. A signal 32 and a pump laser beam 34 are propagating in the core 36 of an Yb/Er doped phosphate glass fiber. The diameter of the core and its refractive index relative to that of the cladding 38 are carefully adjusted such that the fiber core supports only a single mode at the wavelength of the signal. The refractive index of the core has to be higher than that of the cladding to confine the pump and signal beam in the core. A substantial change in the refractive index difference between the core and cladding will change the transverse profile of the signal beam in the core and will affect the propagation of the signal. In particular, if the refractive index in the core is reduced substantially, the confinement of the signal beam can be lost and the fiber amplifier operation will fail. In traditional Yb/Er doped fibers based on silica-based glasses, the concentration of Yb and Er are small and lead to a small absorption coefficient for the pump. Consequently, silica erbium doped amplifiers consist of fibers with a length of several meters and exhibit xcex2 greater than 0. In such fibers, the heating caused by the pump is not critical because the absorption coefficient is really small, hence the change in the refractive index is small. Unfortunately, Yb/Er co-doped silica-based fibers amplifiers are expensive and have large foot print due to the management of the long fiber.
In view of the above problems, the present invention provides a phosphate glass capable of supporting high concentrations of Er and Yb suitable for use in inexpensive compact fiber amplifiers.
This is accomplished with a phosphate glass composition that exhibits a temperature coefficient of refractive index xcex2=dn/dT close to zero. This is achieved by avoiding alkali metal oxides in the glass composition and by adjusting the concentration of the network modifiers such as BaO.
Phosphate glass fibers are an attractive alternative to silica fibers because they can be co-doped with much higher concentrations of Yb and Er and exhibit a higher optical gain per unit length. With such glasses compact and lower cost amplifiers can be fabricated. However, in such fibers the absorption of the pump is high and heating effects caused by the pump laser can cause a significant change in refractive index. This heating effect is even more important when low cost high power multimode pumping is used. In this case, a non-zero temperature coefficient of the refractive index can have an adverse effect on the waveguide mode properties of the signal. Therefore, a temperature coefficient of refractive index xcex2=dn/dT close to zero is desired in phosphate glass compositions for fiber amplifiers.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: